The Actors: Scarlett Johansson, who replaced Samantha Morton in Spike Jonze's sci-fi romance 'Her,' relished turning a disembodied voice into a complex character with a frisky digital presence.

'How I Live Now' | Nov. 8In the near future, an American teen sent to live with her relatives in the English countryside begins adjusting to her new surroundings, until an escalating conflict in Europe changes everything. With Saoirse Ronan, George MacKay and Tom Holland. Written by Jeremy Brock, Tony Grisoni and Jack Thorne. Directed by Kevin Macdonald. Magnolia Pictures (Magnolia Pictures / Magnolia Pictures /June 25, 2012)

Deep in post-production on his weird but wonderful sci-fi romance "Her" this past March, Spike Jonze made the difficult decision to — as the old editing edict goes — kill one of his darlings.

The nuanced vocal performance turned in by British actress Samantha Morton, who was portraying a Siri-esque computer operating system who finds herself in a state of deep romantic longing with Joaquin Phoenix's lovelorn, all-too-human divorced character, in the end "wasn't right" for the film.

And with "Her" set to premiere at the New York Film Festival in October ahead of its Dec. 18 Academy Awards-qualifying run in theaters, drastic action was needed. With Morton's blessing, Jonze — the quirky Oscar-nominated visionary behind "Where the Wild Things Are," "Adaptation." and "Being John Malkovich" — decided to recast the part. "She totally understood we had to go with what the film needed," Jonze said.

Meanwhile, Scarlett Johansson was halfway through her marquee run in a Broadway production of Tennessee Williams' "Cat on a Hot Tin Roof" when Jonze approached her about the role — that of a disembodied voice that must provide the emotional thrust of the film. The actress was daunted by the part's seeming complexity but agreed to an informal rehearsal at Jonze's New York apartment.

That work session evolved into a freewheeling seven-hour discussion of life, love and the "feelings of inadequacy" acutely experienced by Johansson's never-seen character, Samantha, "that stem from the fact she doesn't have a physical body and can't fulfill what a person of the flesh could," Johansson said.

After accepting the part, she and Jonze conducted rehearsals for "Her" concurrent with the actress' "Cat" commitments. "A couple of times, we'd be there on Maggie the Cat's bed going over the scenes," Johansson said. "We wanted the character to be — I almost said 'fleshed out' — there was a certain kind of self-assuredness we needed to develop. That work we did during that time built a wonderful trust between Spike and I. We shared so much of ourselves in that process."

In the movie, Phoenix portrays Theodore Twombly, an emotionally stilted denizen of megalopolis Los Angeles in the not-too-distant future whose job at a company called BeautifulHandwrittenLetters.com entails dictating personal messages for strangers. Theodore's new OS — a frisky digital presence blessed with artificial intelligence, an intense thirst for new experiences and Johansson's dusky vocal timbre — breathes unexpected life into his quotidian existence. And a kind of techno-romance blooms between the two — they go on excursions, vacations, even double dates (with human friends) before unforeseen complications threaten to unplug the couple's connection.

But the movie's ultimate success is predicated on viewers' acceptance of Johansson's disembodied personality as a viable lover — with only a voice capable of emitting affection, jealousy and desire from some nebulous place inside the digital cloud.

In the wake of "Her's" festival debut last month, Johansson's finely calibrated vocal characterization have left critics fumbling for superlatives. "She creates a complex, full-bodied character without any body at all," wrote Variety critic Scott Foundas.

"The character is such a brave one because Samantha allows herself to be vulnerable and completely unaffected," Johansson said recently from Paris, where she was shooting the sci-fi thriller "Lucy" for French auteur Luc Besson. "She doesn't have any preconceived ideas about anything. She's experiencing everything for the first time."

The "Avengers" star recorded most of her performance last spring at a West Hollywood dubbing stage concurrent with filming her part as superheroine Black Widow in "Captain America: The Winter Soldier." Although Phoenix had already wrapped principal photography on "Her" months earlier, when Morton was on board, and was preparing for a role in writer-director Paul Thomas Anderson's new movie at the time, the actor returned for about half of Johansson's vocal sessions, feeding her lines and re-recording some of his own dialogue.

When Johansson traveled with the "Captain America" production to Cleveland in June, Jonze and his microphone followed.

"We'd get her every waking moment off from that other movie, 10 or 12 hours a day," Jonze said. "Months would go by, we'd get her back because we'd rewrite a few lines, finessing it. It was such an exciting way to create characters. You didn't have hair or make-up. It was all about what was being felt."

For her part, Johansson, found the recording sessions challenging but liberating.

"You're liberated from your body. You're liberated from any kind of judgment that other people might place on your appearance," she said. "And the work is so particular. There's such a journey in each scene.